An attempt at explaining injector dead time in a different way, while trying to make it easier to understand what is going on.
What Is Injector Dead Time?
Injector dead time is exactly what it says. It is the amount of time between the point where the ECU told the injector to open and then told it to close, where the injector was not actually open or was not flowing at its peak flow. This occurs because the injector pintle does not move immediately. Things that influence this are:
- Injector solenoid coil charge time. i.e. the time it takes to send energy into the coil and for it to then start moving the pintle. No fuel is injected during this phase.
- Pintle travel time. i.e. the injector flow is not instantly the peak flow, instead, the flow ramps in as the pintle is opening, and ramps shut as it is closing.
What in effect happens is the ECU commands the injector to open. Some time is used up in energising the injector coil, and some more time is used up moving the pintle to the open position. This is lost time where fuel is not being delivered. Some more time is lost as the pintle ramps open, but you do get some fuel. Overall, you did not get all the fuel you asked for.
We also have an extra effect, which is the closing of the pintle. When we command the injector closed it also does not close immediately, so you have a little extra fuel that is injected during this phase. The amount though is far less than is lost during the opening phase.
To visualise this, some poorly made graphs have been created below
What We Think Happens
This is marked in Blue to show the opening time of the pulse and the closing time. This is what the ECU commands when you are tuning the engine.
What Actually Happens
In the below graphs we will try to explain what actually happens with your injector.
Blue = Commanded pulse time
Yellow = Time to energise solenoid before the injector starts to open.
Red = Opening time of the injector
Orange = Closing time of the injector
You can very obviously see that the commanded amount of fuel we want is nowhere near the amount we actually got.
Now, keep in mind that the Yellow, Red, and Orange sections remain the same size no matter what the commanded pulse time is. If we say halved the commanded time we would then find that most of the time the injector is not actually open and is in fact lost. Sure enough, there is a way to correct that.
How To Correct For It?
To correct for this overall loss of fuel what we do is turn the Red triangle into a rectangle with the same area, and do the same for the Orange triangle too. Add the Red rectangle to the end of the pulse, and then subtract the Orange rectangle from it. We subtract the orange section because this was additional fuel we received and not lost like the Red section is.
Now we add to the end of it the missing Yellow section which is the time to energise solenoid before the injector starts to open.
Purple = Injector open time MINUS injector close time
Yellow = Time to energise solenoid before the injector starts to open
We now have one large purple section of missing injector pulse time.
If we now measure the amount of time between the Old End Pulse and the New End Pulse, that is the amount of additional injector opening timing time that needs to be commanded to get the correct amount of fuel delivered to the engine.
That is what is called the Injector Dead Time!
Why Is It So Complicated?
There are a few things that make it more complicated than it first appears.
- The injector opening ramp is not a flat line. It is actually a curve. This makes it difficult to guess.
- The injector closing ramp is also a curve. Same as above, difficult to guess
- This all changes with the voltage applied to the injector. This adds another table dimension to cover for changing battery voltage, and this does have a massive effect.
- It also changes with the fuel pressure. i.e. how much load is placed on the pintle. This adds another table dimension if your fuel pressure varies greatly.
What Happens If The Dead Time is Wrong In My Tune?
If you have incorrect dead time data entered into your ECU you will most likely see the effect where the commanded amount of fuel is the least, i.e. at light load or idle. To demonstrate there are some VE tables shown below.
Normal VE Table
This is how the VE table should look if the injector data is correct.
Dead Time Too Low
When the dead time table is set too low we need to over-compensate in the light load areas by increasing the VE numbers to get more fuel. This very obviously shows in the VE table that something is not set correctly. We must increase the dead time to bring the VE numbers back to normal.
Dead Time Too High
Conversely, if the dead time is set far too high we can create situations where the VE can even reach ZERO as we try to tune the car. Very obviously the VE cannot be zero as some air is passing through the engine, we know immediately that this is not correct and the dead time needs to be decreased.
How To Set The Dead Time If The Data Is Not Available
If no data is available other than possibly the injector flow, which can be measured on a flow machine, then it can be found through experimentation. The simplest way is to not touch the VE table if your engine is relatively standard. and simply adjust the flow rate and dead time until the Air Fuel Ratio (AFR) is fairly close and the vehicle can be somewhat driven normally. When everything is close then fine tune the VE table. Dialling in the dead time and flow rate is a little bit of a see-saw practice, moving one and then moving the other, and repeating until the adjustments are small.
Some things like camshaft timing greatly impact the efficiency of the engine which can also skew results. In general the larger the valve overlap the lower the engine vacuum at idle and the lower the VE of the engine at idle. As a rough guide the following is the expected VE for an engine at idle. This can be used as a starting point to evaluate how much adjustment is needed to dial in the dead time. The more efficient an engine is the less air is needed, and to supply less air the throttle is closed more, which in turn creates more vacuum (rather than more RPM). So the higher the vacuum the more efficient the engine is for that amount of Manifold Pressure.
Idle Engine Vacuum kPa
Approximating the Flow Rate
After taking an educated guess at the flow rate and dead time so that the engine is running and idling without having touched the VE table, the next step is to get a little closer to the flow rate. To do this we need to bring the engine load and rpm up to a level where the dead time has less effect and the flow rate has a significant effect. We pretty much need the engine at full load and keep the rpm above idle but low enough that the engine is not being beaten on too hard. Aim for around 1500 to 2500 rpm for this. While under load is the AFR is trending lean, then you need to adjust the flow rate down. If you are too rich you need to adjust the flow rate up. Easy as it gets!
Approximating the Dead Time
After the flow rate has been adjusted, now when you come back to idle the AFR will have changed. Now adjust the whole table up or down to bring the AFR back to your Target. After this is complete go back and retest the flow rate again, then repeat the dead time adjustment one more time.
After you have the dead time worked out, the next step is to vary the voltage. The easiest way is to disable the Alternator and allow the voltage to drop. Typically the engine will be at around 14V with the alternator running and without it, you can let the voltage drop down to 12V. This will give you 2x point from which to take a good solid guess at the curve. In reality, if you are at 10V and still driving you probably have far bigger concerns than your fuelling being off by a couple of percent, and as such fussing about small changes from here is usually counterproductive for most applications! If it is of concern for you, then you need to find a way to let your engine run at lower and lower voltages all the way down until the sensors all give up and the engine shuts off.
Q: My car won't start, is it the Dead Time that needs adjusting?
A: Nope. It is most likely not the cause of your engine not starting.
Q: My car is not making the power I expect, do I need to adjust the Dead Time?
A: Nope. If the AFR and the Timing are correct then the lack of power is mechanical.
Q: My car is slow to build boost, is it the Dead Time table I need to look at?
A: Nope. It has no impact on a turbo's ability to build boost.
Q: My car is running very rough and we can't tune it out, it must be the Dead TIme?
A: Nope. If normal adjustment of fuelling does not solve it then the problem is elsewhere.
Q: My VE table looks weird but the engine runs fine, is the cause my Dead Time table?
A: Yes, most likely. Good work picking up on that!
Q: My AFRs are a little unstable at extremely light load and tuning is very difficult, is it the Dead Time table?
A: Most likely not, it is more that you are likely running into Low Pulse Width issues where the injector does not completely open before it is commanded to close, and that is a topic for another day.